As is well known, various alkaline storage batteries (rechargeable batteries) are used as a power supply for portable electronic equipment and for electric vehicles, hybrid electric vehicles and the like. One type of the alkaline storage batteries is a nickel-metal hydride storage battery which are high in energy density and excellent in reliability. The nickel-metal hydride storage battery includes a cathode containing nickel hydroxide as a principal component, an anode containing a hydrogen-absorbing alloy as a principal component, and an alkaline electrolyte containing potassium hydroxide or the like.
As a nickel-metal hydride storage battery for a vehicle, there is known a rectangular, sealed storage battery in which a plurality of electric cells are electrically connected in series to generate a certain power capacity. FIG. 3 illustrates a partial sectional view of such rectangular, sealed storage battery.
The rectangular, sealed storage battery illustrated in FIG. 3 includes a plurality of battery cases 130 each forming an individual battery cell 110 that is a nickel-metal hydride storage battery. The plurality of battery cases 130 are connected together via partitions 120 and housed inside a rectangular, integral battery case 100. Upper openings of the battery cases 130 are sealed by a lid 200. Inside each battery cell 130, a power generation element, made up of an electrode plate group 140, formed by laminating cathode plates and anode plates via a separator, and current collectors 150 and 160, respectively joined to opposite sides of the electrode plate group 140, is housed together with an electrolyte. A lead portion 141a, projecting to a side from each cathode plate of the electrode plate group 140, is joined to the current collector 150, and a lead portion 141b, projecting to a side from each anode plate of the electrode plate group 140, is joined to the current collector 160. The current collectors 150 and 160 respectively include connection projections 151 and 161. By the connection projections 151 and 161 of the current collectors 150 and 160 of adjacent battery cases 130 being electrically connected together via a through hole 170 in the partition 120, the adjacent battery cases 130 are electrically connected in series. The total output of the plurality of battery cases 130, that is, the plurality of battery cell 110 that are thus connected in series is taken out from a cathode or anode connection terminal TM provided in the through hole 170 of an end side wall of the integral battery case 100. Besides the above, in the present rectangular, sealed storage battery, a safety valve 210 for releasing the internal pressure in the integral battery case 100 when the pressure becomes no less than a fixed value, a sensor installation hole 220 for installing a sensor for detecting the internal temperature of the battery, etc., are provided in the lid 200.
Hydrogen is generated during charging of a nickel-metal hydride storage battery, and the generation of hydrogen gas is suppressed by hydrogen absorption by the hydrogen-absorbing alloy. To prevent the hydrogen gas that cannot be absorbed by the hydrogen-absorbing alloy from increasing the internal pressure of the storage battery, the capacity of the anode is made no less than 1.5 times the capacity of the cathode to increase the rate of hydrogen absorption by the hydrogen-absorbing alloy in many cases. However, in this case, the capacity portion of the anode capacity that is in excess of the cathode capacity is a wasted capacity that is not used by the storage battery. An example of a nickel-metal hydride storage battery by which such waste of capacity of the anode can be reduced is described in Patent Document 1.
The nickel-metal hydride storage battery described in Patent Document 1 is provided with a cathode including an active substance containing nickel hydroxide as a principal component and an anode including a hydrogen-absorbing alloy. A composition ratio of five metals making up the hydrogen-absorbing alloy of the anode is set to a certain ratio to increase hydrogen absorption rate of the hydrogen-absorbing alloy. That is, the rate of hydrogen absorption by the hydrogen-absorbing alloy of the anode is increased so that the hydrogen generated during charging is absorbed rapidly and the generation of hydrogen gas from the anode is suppressed. The capacity of the anode of the nickel-metal hydride storage battery is thereby suppressed to approximately 1.1 to 1.5 times the capacity of the cathode and increase of the internal pressure of the storage battery is also suppressed.